Deformed soft-sediment deformation structures (SSDS) can indicate polyphase deformation events and provide valuable insights into the inversion process of a basin. Herein, we present the Miocene–Quaternary deformation inversion history of the Bomun sub-basin in the Gyeongju area of SE Korea. The inferred ENE compression direction (σ_Hmax) based on paleostress analysis of the fault system, displacing Miocene sediments and SSDS, corresponds to the current stress field. The widespread occurrence of clear liquefaction structures and the vertical repetition of SSDS indicate substantial seismic activity during the basin opening stage. Brittle deformation features observed at both outcrop- and microstructural-scale along the faults suggest a reactivation as reverse faulting associated with a tilting process. The tectonic history of the study area is distinguished by SSDS associated with seismic activity, and reverse faulting associated with inversion process under ENE oriented σ_Hmax. The Environmental Seismic Intensity Scale (ESI-07) based on the SSDS indicates seismic intensity of IX-X, which might be related with the opening of the Bomun sub-basin. Therefore, detailed analyses of SSDS could provide valuable insights on the dynamics of local geology and contribute to further extensive research on seismic hazards and basin inversion.